Add Steam Audio 4.8.1 spatializer plugin

Official Unity package under Assets/Plugins/SteamAudio; enabled as
Spatializer + Ambisonic Decoder in the audio settings (HRTF: sounds
behind the listener are filtered/muffled directionally). Download
staging Tools/SteamAudio/ gitignored.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Plugins/SteamAudio/Scripts.meta

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